*3.1.2 Amino acids*

Although the light-struck aroma precursors are only cysteine and methionine, the analysis methods found in the literature do not focus solely on these amino acids but consider as much of them as possible. Among the different techniques found in literature, the HPLC is the most frequently used for the determination of these compounds in wine or must. However, since the amino acids have no a specific chromophore group to be detected, a derivatization step is necessary. Although this derivatization can be performed before or after chromatographic separation, the pre-column option followed by HPLC or UPLC has been more widely used due to its simplicity and versatility. This derivatization reaction can be performed by using several reagents which lead to derivatives detectable by different detectors. Thus, when using ultraviolet detector, derivatising reagents such as phenylisothiocyanate, diethyl ethoxymethylenemalonate or dansyl chloride can be used. The latter reagent can also be used with fluorescence detectors in addition to other such as *o*-phthaldialdehyde, 9-fluorenylmethylchloroformate or 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate. All of them present some advantages and some drawbacks but, in all cases, the main problem is that the time spent on the analysis is long [8, 64]. This is why some authors have focused on developing faster methods, such as the use of a fully automated in-loop derivatization procedure [65]. However, although these methods have been successfully applied, the way to drastically reduce the analysis time has been achieved when the derivatization step has been avoided. Today the only technique that allows a high degree of sensitivity and selectivity in the determination of amino acids without derivatization is the

so-called liquid chromatography with tandem mass spectrometry (LC–MS–MS), a mass spectrometer system highly specific for each compound structure. However, it should be pointed out that this costly technique has been applied very little to the analysis of wine components, and even less to the amino acid analysis of wine.

### **3.2 Analysis of volatile compounds**

The volatile sulfur compounds related to LST constitutes a chemical family that includes thiols, sulfides and disulfides. This structural diversity together with the highly reactive nature of these compounds, their low volatility and their low concentration in a matrix as complex as wine, make their analysis considerably difficult.

Although the older bibliography references show methods developed as early as the 1990s which used sulfur-specific ion electrodes or the spectrophotometry with previous treatments to trap sulfur compounds but these methods have been rendered obsolete [66, 67]. In fact, nowadays, the best results are obtained when using gas chromatography coupled to specific detectors so this is the most widely used technique to analyze these compounds. Flame-photometric (FPD) [68], sulfur chemiluminescence (SCD) [69] and more recently pulsed-flame-photometric [70] are the usual required detectors. The use of the mass spectrometry (GC–MS), even being a nonspecific detection system, can be a good option mainly when working with SIM mode as it confers better sensitivity.

In any case, taking into account the usual low concentrations and the highly reactivity of VSCs, a preconcentration technique with minimal manipulation of the sample is required prior to chromatographic separation. Thus, while liquid–liquid extraction systems (either with vacuum or using reagents that selectively trap thiols such as pHMB) have not been very successful, the application of the headspace technique has given very good results. It should be noted that the concentration process required is only achieved with the dynamic modality of this technique which is also called purge and trap technique. Among the different traps, the best results are obtained when working with cold traps because, when dealing with chemical traps and complex matrices such as wine, the so-called memory effect usually occurs due to the difficulty of cleaning the traps between analyses. More recently, the technique that has emerged as the most appropriate is the so-called solid-phase microextraction (SPME) which is applied to the headspace of the sample wine. This simple and fast technique involves immersing a polymer-coated fiber into the headspace sample to extract and concentrate the analytes on the fiber. The fiber coatings that provides the best results on the sulfur compounds extraction have been Carboxen/polydimethylsiloxane or divinylbenzene/Carboxen/ polydimethylsiloxane. Regarding the variables that influence the extraction process, the literature indicates that it is necessary to increase ionic strength with sodium chloride or magnesium sulfate, to agitate the sample with slow-medium speed and to use extraction temperature and time between 35 and 40°C and 20 and 40 minutes, respectively [37, 68].

#### **4. Prevention and correction measurements**

Up to now, different preventive and corrective measures have been studied to avoid the onset of the LST in both still and sparkling wines. Thus some proposed preventive measures are: avoiding grapes treated with sulfur on dates close to the harvest, avoiding excessive sulfite in grape juice, use yeast strains and nutritional conditions with low production of aromatic precursors of VSCs, racking wines

#### *The Light Struck Taste of Wines DOI: http://dx.doi.org/10.5772/intechopen.99279*

correctly, use micro-oxygenation or use preventing agents, such as polyphenols [5, 21]. Regarding to the possible corrective measures to reduce or eliminate VSCs, these could be: aerating the wine, using colloidal copper or copper sulfate or using insoluble absorbing materials such as bentonite, active carbon, or charcoal. In any case, whatever preventive or corrective measure has been chosen, it must be carefully applied as he inadequate practice of these treatments can lead to a great loss of the organoleptic characteristics of wines.
